1. Field of the Invention
The present invention relates to new anisotropic compounds, i.e. a new class of cyclohexyl cyclohexylmethyl ethers, as well as to liquid crystal (LC) mixtures containing the cyclohexyl cyclohexylmethyl ethers used as the dielectric medium for liquid crystal displays. The invention further relates to a method of producing the new anisotropic compounds.
2. Description of the Prior Art
Anisotropic biphenyl compounds of the formula (10): ##STR3## in which R.sup.1 means an alkyl radical, for example, are known from the German Offenlegungsschrift No. 2,356,085 and are among the substances most commonly used for LC-mixtures.
Furthermore, it is known from the German Offenlegungsschrift No. 2,636,684 that the relatively high viscosity and optical anisotropy (.DELTA.n) of the compounds of formula (10) can be reduced by substituting a trans-1,4-cyclohexylene ring for a phenylene ring, which leads to phenylcyclohexanes of the formula (20): ##STR4## in which R" has the same meaning as R' in formula (10).
The .DELTA.n values of anisotropic components of LC-mixtures customarily represent a measurement of the optical anisotrophy and are expressed as the difference, .DELTA.n=n.sub..parallel. -n.sub..perp., of the refracting indices parallel or perpendicular, respectively, to the molecular axis of the substance being considered.
For many types of LC-displays, particularly the so-called "guest-host" effect cells, nematic LC-phases with as small .DELTA.n-values as possible, e.g., not more than 0.1, are needed. By replacing one phenylene ring in compounds of formula (10) with a trans-cyclohexylene ring, the .DELTA.n-value is practically reduced by one-half (.DELTA.n.apprxeq.0.22 with compounds of formula (10), n 0.12 with compounds of formula (20)).
If the second phenylene ring of the anisotropic compounds of formula (10) or the remaining phenylene ring of the compounds of formula (20) is replaced by a cyclohexylene ring, one arrives at the cyclohexylcyclohexanes of formula (30) known from the German Offenlegungsschrift No. 2,702,598, ##STR5## in which R" has the same meaning as that given for R' in formula (10); also, the .DELTA.n-value is thereby reduced to values less than 0.1, typically to ca..DELTA.n=0.06.
The disadvantage, however, with the compounds of formula (30) is that hydrogenation of the second phenylene ring causes a definite tendency toward forming smectic phases (see R. Pohl et al, Phys. Letter, 65A (2), 169(1978).
The applicant suggested in the Swiss patent application No. 6672/79 (also see M. A. Osman and L. Revesz, Mol. Cryst. and Liqu. Cryst., 56 105(1979)) a new compound class, namely cyclohexyl cyclohexanoates of formula (40): ##STR6## in which X' and Y' independently represent hydrogen, alkyl group with 1 to 12 C-atoms, alkoxy group with 1 to 12 C-atoms, monoalkylamino group with 1 to 12 C-atoms, halogen, nitrite (--CN), nitro, or para substituted cyclic radical, e.g., a phenyl, cyclohexyl, piperidine or pyrrolidine ring that is connected by way of a simple covalent bond, a carboxyl, or oxycarbonyl group with the cyclohexyl rings (A) and (B) of the basic skeleton of formula (40), with the requirement that only one of the groups X', Y' means a cyclic radical.
The anisotropic compounds of formula (40) have advantageously low .DELTA.n-values, chiefly less than 0.1, and, in contrast to the known compounds of formula (30), have no definite tendency to form smectic phases. However, the carboxyl group forming the bridge between cyclohexyl rings (A) and (B) of compounds of formula (40) is relatively reactive and sensitive to hydrolysis.